Voltage drop from leisure battery to lighting

[theflyingpig]

I recently bought a semi-converted-to-camper van and am trying to understand the wiring....

Something ain't right! Multimeter reading 12v ish at leisure battery, but only 1.5v at an internal light...

Wiring all hidden behind lining..

Anyone know how best to proceed....?

 

[Jurassicspark]

That 1.5v may be a phantom voltage.
Best way would be to check continuity from both ends.